This invention relates to methods and apparatus for handling fluids in a two-phase flow, pipeline system, such as, gas and liquid, or two immissible liquids of different specific gravities. This invention particularly relates to a method and apparatus for transferring liquids from the suction side of a gas compressor station to the discharge side of such station whereby the liquid does not enter or impair the operation of the compressors in the gas compressor station. However, this invention is also useful as a method and apparatus for withdrawing liquids from a two-phase flow pipeline not having a compressor station at the point of withdrawal, for storing the liquids, and for reinserting the liquids at a suitable later time back into the same pipeline or into another pipeline of suitable character.
The accumulation of liquid hydrocarbons in natural gas pipelines connected to flowing production wells creates problems if the accumulated liquid reaches the compressors in a gas compressor station installed in such pipeline. When a large diameter pipeline system is purposely utilized to transport large quantities of liquids intermingled with gas and the pipeline is adapted to utilize a movable sphere for urging the liquids along the pipeline, it is not uncommon for a three or four thousand barrel slug of liquid to arrive at a gas compressor station, thus presenting a difficult problem of transferring the liquid out of and back into the system to avoid the gas compressors. When the two-phase fluids are transported at high pressures and velocities, the problem of handling the liquid at a gas compressor station becomes even more difficult.
Historically, removal of accumulated liquid from a two-phase flow pipeline system at a gas compressor station has been accomplished through the use of separators, storage tanks, and pumps for inserting the liquid back into the pipeline system on the discharge side of the gas compressor station. This traditional prior art manner of handling liquids at a gas compressor station has been the practice for gas compressor stations built on land because large storage tanks can be constructed adjacent the compressor station. This traditional manner of handling liquids at a gas compressor station has on occasion also been used for offshore and onshore pipeline systems which do not employ a sphere or "pig" for accumulating and transferring the liquid, as the separators in such systems can handle relatively small quantities of liquid arriving at the station. But the traditional methods are disadvantageous when large slugs of liquid arrive at the station in a spontaneous manner. The prior art systems are particularly disadvantageous when the compressor station is located on a platform in offshore waters where it is difficult and costly to construct large storage tanks at the compressor station or when the two-phase pipeline transmission system utilizes spheres or "pigs" for periodically accumulating and sweeping the liquids along the pipelines whereby large slugs of liquid are regularly brought to the gas compressor station.
It is an object of this invention to provide an improved method and apparatus for transferring liquids being transported in a two-phase flow pipeline system from the suction side to the discharge side of a gas compressor station in such system, which method and apparatus is particularly adaptable for use in offshore environments.
It is a further object of this invention to provide a method and apparatus for transferring selected quantities of liquid flowing in a two-phase flow pipeline system from the suction side to the discharge side of a gas compressor station in such system and which method and apparatus may be utilized to transfer large volumes of liquid being transported at high pressures and velocities.
It is still a further object of this invention to provide a method and apparatus for separating large quantities of liquids flowing at high pressures and velocities in a two-phase flow pipeline whereby such liquids can be removed from the pipeline system at line flow velocities and whereby such liquids can be reinserted into the pipeline system at selected velocities up to line flow velocities.
It is an object of this invention to provide an improved method and apparatus for transferring at line velocities selected quantities of liquid flowing in a two-phase flow pipeline system from the suction side to the discharge side of a gas compressor station in such system without the use of special pumps or atmospheric or high pressure storage tanks.
In the method and apparatus according to this invention, as applied to a liquid/gas pipeline system at a point where there is a gas compressor station, a slug of liquid or accumulated liquid arriving at the gas compessor station is directed into a separator means on the suction side of the compressor station. Any incoming gas which is mixed with the liquid or is transported ahead of the liquid is bled off the separator upwardly through a control valve and is thereafter compressed by the compressor station in the normal manner. The liquid is removed from the lower portion of the separator. The liquid level in the separator is detected and a controller effects opening or closing of the control value to control the differential pressure across the accumulated liquid whereby the rate of flow of the liquid out of the separator is controlled. The liquid flowing out of the separator is fed into an elongated accumulator loop which, in the case of use in an offshore environment, may be constructed on the sea bed. Preferably the elongated accumulator loop has previously been filled with gas. A sphere is positioned into the accumulator loop to separate the liquid from the gas downstream in the accumulator loop. As the liquid moves into the accumulator loop, the gas stored in the accumulator loop is forced out, combined with any gas bled off the separator through the control valve, and fed to the compressor in the gas compressor station to provide such compressor with a continuous source of gas to be compressed. When a selected quantity of liquid has been transferred into the accumulator loop, selected valving is operated to supply pressure from the discharge side of the gas compressor station into the accumulator loop to drive the liquid out of the accumulator loop and into the pipeline on the discharge side of the compressor station.